Faculty Research Page

Elçin Ünal

Research Interests

Gametogenesis is a developmental program in which a precursor germ cell undergoes division and differentiation to produce mature germ cells, termed gametes. A fundamental question is how the fitness of gametes is ensured during their production such that they contain the appropriate nuclear and cytoplasmic content to make healthy progeny. For instance, when cellular damage is present in germ cells, do mechanisms exist to prevent transmission to the gametes? How is the precise genetic complement distributed to the gametes? Using the genetic organisms S. cerevisiae and C. elegans, we are interested in addressing these and related questions by studying gametogenesis in two fundamental frameworks: (1) in the context of aging, with the aim of understanding how gametogenesis counteracts age-induced cellular damage and (2) in the context of meiotic chromosome segregation, with the aim of identifying the mechanisms that ensure correct partitioning of genetic material to produce healthy progeny. Both questions have broad reaching impact on human health, including the development of novel strategies to combat age-induced cellular damage and uncovering the basis of genome partitioning defects associated with maternal age.

Current Projects

Project I ­– Effects of Gametogenesis on Aging and Age-induced Cellular Damage

A. Gametogenesis resets lifespan by eliminating age-induced cellular damage in S. cerevisiae.
Although eukaryotic organisms age, the detrimental age-associated traits are not passed onto the next generation. To understand how the progeny is devoid of age-induced defects, we asked whether cellular rejuvenation occurs during gametogenesis. Budding yeast provides unparalleled tools to address this question: First, gametogenesis, known as sporulation, is easily induced. Second, yeast cells undergo replicative aging as each cell produces a finite number of progeny. Finally, both young and aged cells can be isolated easily and monitored throughout gametogenesis.

We found that the lifespan of the gametes from young and aged cells were indistinguishable from each other. In contrast, aged cells obtained by the same procedure but not induced to undergo gametogenesis died rapidly (Figure 1). Furthermore, gametogenesis-induced rejuvenation is associated with the loss of age-induced cellular damage. Examples include the extrachromosomal circles that arise from illegitimate recombination at the ribosomal DNA locus, protein aggregates that associate with the chaperone Hsp104 and morphological aberrations of the nucleoli. We observed that gametes no longer contained any of the cellular defects that were present in the aged precursors (Figure 2). Thus, in yeast, gametogenesis resets lifespan, likely by eliminating age-induced cellular damage.

B. Ndt80, a gametogenesis-specific transcription factor, extends lifespan in mitotic cells.
We further found that the transient and ectopic expression of a gametogenesis-specific transcription factor, NDT80, in mitotic cells was sufficient to extend the lifespan of aged cells and to restore nucleolar morphology. This result indicates that at least a subset of the factors required for rejuvenation during gametogenesis could function outside of this developmental program to reverse age-induced cellular damage, thereby providing a feasible strategy to identify them.

C. Development of C. elegans as a metazoan model system to study gametogenesis-induced rejuvenation.
To study the evolutionarily conserved and diverged features of gametogenesis-induced rejuvenation, we have recently extended our studies to C. elegans. Aged worms, like yeast, display increased proteotoxic stress and nucleolar aberrations. We recently began to build tools for live imaging of the nucleoli by generating transgenic worms that contain a fluorescently tagged nucleolar protein.

We also initiated studies on cep-1, the closest relative of NDT80 in worms. CEP-1 is homologous to TAp63α, a p53 family member, which plays an important role in oocyte quality control in mice. In adult worms, cep-1 is highly expressed in the germline, though cep-1 function has so far been analyzed mainly in the context of apoptosis, based on its homology to the p53 family. However, we favor the notion that the ancient function of the p53 family members, including Ndt80, CEP-1 and TAp63α, is to promote quality control during gametogenesis. We speculate that this primitive function of the p53 family was later modified to accommodate general stress response and tumor suppressive functions.

Remaining key questions include:

What changes to the nucleolus occur during aging that may be limiting to lifespan?

Which downstream targets of NDT80 promote rejuvenation and how do they function?

How does gametogenesis counteract cellular damage in higher eukaryotes?

A. Timing of microtubule-kinetochore interactions defines meiosis I chromosome segregation.In meiosis, two consecutive rounds of nuclear division called meiosis I and meiosis II follow a single round of DNA replication to produce haploid gametes. During meiosis I (MI), homologous chromosomes segregate. Meiosis II resembles mitosis in that sister chromatids split. Proper meiotic chromosome segregation requires three key events that modulate how chromosomes interact with each other and with the microtubule cytoskeleton: (1) recombination between homologous chromosomes, (2) the way linkages between sister chromatids, known as sister-chromatid cohesion, are removed from chromosomes and (3) the manner in which chromosomes attach to the meiotic spindle.

We discovered a fourth key event essential for proper meiotic chromosome segregation, temporal restriction of microtubule-kinetochore interactions (Figure 3). We found that microtubule-kinetochore interactions must be inhibited during premeiotic S phase and prophase I to allow for two aspects of MI chromosome morphogenesis: the assembly of a protected centromeric chromatin domain and the association of factors with sister kinetochores to enable attachment to microtubules emanating from the same pole (monopolar attachment). We found that when microtubules prematurely interact with kinetochores, sister kinetochores attach to microtubules emanating from opposite poles (bipolar attachment). This geometry, in turn, interferes with MI chromosome morphogenesis and segregation, eventually leading to the transformation of MI into a mitosis-like division (Figure 4).

We further defined the mechanism whereby premature microtubule-kinetochore interactions are inhibited prior to MI. The inhibition occurs through regulation of cyclin-CDK activity and of outer kinetochore assembly. Our results demonstrate that preventing premature microtubule-kinetochore interactions is essential for establishing a MI-specific chromosome architecture and provide critical insights into how the mitotic chromosome segregation machinery is modulated to achieve a MI-specific pattern of chromosome segregation.

B. Cyclin-CDKs differentially regulate microtubule-kinetochore interactions in meiosis.
In budding yeast, a single cyclin-dependent kinase (CDK) associates with one of five B-type cyclins in meiosis. Interestingly, misexpression of only a subset of cyclins causes stable microtubule-kinetochore interactions in prophase I. The different phenotypes do not correlate with total cyclin-CDK activity, strongly indicating substrate specificity among the meiotic cyclin-CDKs with respect to promoting stable microtubule-kinetochore interactions. Identification of these differential substrates will provide important mechanistic insights into cyclin-CDK dependent regulation of microtubule-kinetochore interactions and chromosome segregation during meiosis.

C. Protection of centromeric cohesion in meiosis I requires additional regulatory events. In anaphase I, cohesin, the protein complex that mediates sister chromatid cohesion, is removed by cleavage of a subunit from chromosome arms to allow homologs to segregate. However, cohesin around the centromeres is protected from cleavage until anaphase II by the Sgo1/PP2A complex. Interestingly, we found that premature MT-KT interactions cause a defect in centromeric cohesin protection that is distinct from defects in localizing Sgo1/PP2A complex (Figure 5).

Presence of Sgo1/PP2A around the centromeres is thought to be the sole requirement for centromeric cohesin protection. Our finding strongly indicates that the centromeric cohesin protection requires additional events. Further analysis can provide insights into the cause of maternal-age induced chromosome missegregation, since defects in cohesion maintenance has been implicated in age-dependent meiotic malfunctions in mammals.

Remaining key questions include:

What are the molecular targets of cyclin-CDKs that promote stable microtubule-kinetochore interactions?

What are the regulatory events that mediate centromeric cohesin protection?

How is kinetochore assembly and function regulated during meiosis?

Movie description: This clay animation describes the process of meiotic chromosome segregation, and highlights how premature microtubule-kinetochore interactions transform meiosis I into a mitosis-like division. (Special thanks to (1) Dr. Matthew P. Miller, for his overall help in making this movie, especially for his dedication of taking over 300 pictures and trying numerous musical pieces (2) Jingxun Chen, for her help in constructing each picture scene)